Blends of polystyrene/polypropylene grafted polymers and elastomeric tetrablock copolymers

ABSTRACT

Compositions comprising a polypropylene grafted with polystyrene and a selectively hydrogenated monoalkenyl aromatic/conjugated diene block copolymer have improved melt viscosity and toughness when the block copolymer component is designed to have weak monoalkenyl aromatic domains.

FIELD OF THE INVENTION

The invention relates to compositions that contain grafted polypropylenepolymers and elastomeric block copolymers. More specifically, theinvention relates to blends of styrenic block copolymers andpolypropylene that is grafted with polystyrene.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,990,558 describes graft-copolymer-based rigidthermoplastic compositions comprising, by weight: (a) about 60-95% byweight of the polymer components of the graft copolymer comprising about10 -65 % by weight of the graft copolymer of a styrene polymer graftedonto a backbone of propylene material, and, complimentary (b) about40-5% by of the polymer components of a rubber component comprisingabout from 20-100% by weight of the rubber component least onemonoalkenyl aromatic hydrocarbon-conjugated diene block copolymer, (ii)at least one block copolymer which is a hydrogenated product of (i), or(iii) a mixture of at least one (i) block copolymer with at least one(ii) block copolymer, and (2) from about 80-0% by weight of the rubbercomponent of an olefin copolymer rubber, e.g., EPM (ethylene-propylenemonomer rubber). Optionally this formulation can contain 5-30 parts of apropylene polymer material per 100 parts of the graft copolymer andrubber component.

Modifying graft copolymers of styrene polymers on substrates ofpropylene polymer material by blending with a monoalkenyl aromatichydrocarbon-conjugated diene block copolymer was found to improve theimpact strength and ductility of the graft copolymers. Combinations ofblock copolymer and olefin copolymer rubbers gave a greater improvementin the graft copolymer's impact strength and ductility than was observedfor the block copolymer alone. The examples describe thermoplasticcompositions that contain block copolymers having the structurepolystyrene-hydrogenated polybutadiene-polystyrene (S--EB--S) orpolystyrene-hydrogenated polyisoprene (S--EP). The block copolymer isbroadly defined to include S--EP--S and S--EB structures as well asbranched and radial variations of the block copolymers.

SUMMARY OF THE INVENTION

Compositions comprising from 60% to 95% by weight of the polymercomponents of a grafted polypropylene and from 40% to 5% by weight ofthe polymer components of a rubber component comprising a selectivelyhydrogenated styrene/diene block copolymer have significantly improvedmelt viscosity and toughness when the block copolymer component isdesigned to have weak styrene domains.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a rigid thermoplastic compositioncomprising about 60-95% by weight of the polymer components of a graftcopolymer comprising about 10-65% by weight of the graft copolymer of astyrene polymer grafted onto a backbone of propylene material, and,about 40-5% by weight of the polymer components of a rubber componentcomprising (1) from about 20-100% by weight of the rubber component of aselectively hydrogenated block copolymer which has weak styrene domainsand from about 80-0% by weight of the rubber component of an olefincopolymer rubber, e.g., EPM (ethylene-propylene monomer rubber). Therigid composition has significantly improved toughness and meltviscosity in comparison to similar compositions containing differentstyrenic block copolymers.

Preferably the selectively hydrogenated block copolymer is selected froma group of block copolymers having weak monoalkenyl aromatic domains,the group comprising tetrablock copolymers having the structuremonoalkenyl aromatic-hydrogenated conjugated diene-monoalkenylaromatic-hydrogenated conjugated diene, asymmetric triblock copolymershaving the structure monoalkenyl aromatic-hydrogenated conjugateddiene-monoalkenyl aromatic wherein one monoalkenyl aromatic block issignificantly greater in peak MW than the other monoalkenyl aromaticblock, and symmetric triblock copolymers having the structuremonoalkenyl aromatic-hydrogenated isoprene-monoalkenyl aromatic.

The major component in the blends of the present invention is a styrenicgrafted propylene polymer material as described in U.S. Pat. No.4,990,558, which is incorporated by reference herein. Styrenic graftedpropylene polymer material as used in this specification means thosegrafted propylene polymer materials where the styrenic grafting monomeris styrene, an alkyl ring-substituted styrene where the alkyl is methylor ethyl, combinations thereof wherein the alkyl ring-substitutedstyrene is present in an amount of from 5 to 95%, or a combination ofstyrene or an alkyl ring-substituted styrene with 5 to 40% ofalphamethyl-styrene or alpha-methyl-styrene derivatives. Preferredgrafting monomers are styrene and mixtures of styrene andalpha-methylstyrene.

The propylene polymer material backbone of component (b) can be (i) ahomopolymer of propylene, (ii) a random copolymer of propylene and anolefin selected from ethylene and C₄ -C₁₀ alpha-olefins, provided that,when the olefin is ethylene, the maximum polymerized ethylene content isabout 10%, preferably about 4%, and when the olefin is a C₄ -C₁₀alpha-olefin, the maximum polymerized content thereof is about 20%,preferably about 16%, (iii) a random terpolymer of propylene with twoalpha-olefins selected from the group consisting of ethylene and C₄ -C₁₀alpha olefin, provided that the maximum polymerized C₄ -C₈ alpha-olefincontent is about 20%, preferably about 16%, and when ethylene is one ofsaid alpha-olefins, the maximum polymerized ethylene content is about5%, preferably about 4%, (iv) a homopolymer of (i) or a random copolymerof (ii) impact-modified with from about 5 to 30% of anethylene-propylene copolymer rubber, or anethylene-propylene-non-conjugated diene monomer rubber having a dienecontent of about 2 to 8%. The ethylene content of (iv) being from about20 to about 70%. The C₄ -C₁₀ alpha-olefins include linear and branchedC₄ -C₁₀ alpha-olefins such as 1-butene, 1-pentene, 4-methyl-pentene-1,3-methyl-1-butene, 1-hexene, 3-4-dimethyl-l-butene, 1-heptene,3-methyl-1-hexene, 1-octene and the like.

Suitable particulate forms of the grafted propylene polymer materialinclude powder, flake, granulate, spherical, cubic and the like.Spherical particulate forms prepared from a propylene polymer materialhaving a pore volume fraction of at least about 0.07 are preferred.

The styrene monomer, alkyl ring-substituted styrene monomer ormethylstyrene derivative, except alpha-methylstyrene, can be used aloneor in combination with each other or with alpha-methylstyrene to graftonto the propylene polymer material backbone. All exceptalpha-methylstyrene and its derivatives readily homopolymerize when usedalone and graft copolymerize with each other when two or more arepresent, including alphamethyl-styrene.

Preferred styrenic grafted propylene polymer materials of the presentinvention are grafted copolymers of styrene or of styrene and amethylstyrene derivative monomer on polypropylene or on animpact-modified polypropylene backbone.

The styrenic grafted propylene polymer material of the present inventionis prepared by the free-radical-initiated graft polymerization of atleast one vinyl monomer, at free radical sites on propylene polymermaterial. The free-radical sites may be produced by irradiation or by afree-radical generating chemical material, e.g. by reaction with asuitable organic peroxide.

According to the method where the free-radical sites are produced byirradiation, the propylene polymer material, preferably in particulateform, is irradiated at a temperature in the range of about 10° to 85° C.with high-energy ionizing radiation to produce free-radical sites in thepropylene polymer material. The irradiated propylene polymer material,while being maintained in a substantially non-oxidizing atmosphere, e.g.under inert gas, is then treated at a temperature up to about 100° C.for a period of at least about 3 minutes, with about from 5 to 80% ofthe particular grafting monomer or monomers used, based on the totalweight of propylene polymer material and grafting monomer(s). After thepropylene polymer material has been exposed to monomer for the selectedperiod of time, simultaneously or successively in optional order, theresultant grafted propylene polymer material, while still maintained ina substantially non-oxidizing environment, is treated to deactivatesubstantially all of the residual free radicals therein, and anyunreacted grafting monomer is removed from said material.

The free-radical deactivation of the resulting graft copolymer isconducted preferably by heating, although it can be accomplished by theuse of an additive, e.g. methyl-mercaptan, that functions as afree-radical trap. Typically the deactivation temperature will be atleast 110° C., preferably at least 120° C. Heating at the deactivationtemperature for at least 20 minutes is generally sufficient.

Any unreacted grafting monomer is removed from the graft copolymer,either before or after the radical deactivation, or at the same time asdeactivation. If the removal is effected before or during deactivation,a substantially non-oxidizing environment is maintained.

In the method where the free-radical sites are produced by an organicchemical compound, the organic chemical compound, preferably an organicperoxide, is a free-radical polymerization initiator which has adecomposition half-life of about 1 to 240 minutes at the temperatureemployed during the treatment. Suitable organic peroxides include acylperoxides, such as benzoyl and dibenzoyl peroxides; dialkyl and aralkylperoxides, such as di-tert-butyl peroxide, dicumyl peroxide, cumyl butylperoxide, 1,1-di-tert-butylperoxy-3,5,5-trimethylcyclohexane,2,5-dimethyl-2,5-di-tert-butylperoxyhexane, andbis(alpha-tert-butylperoxyisopropylbenzene); peroxy esters, such astert-butylperoxypivalate, tert-butyl perbenzoate, 2,5-dimethylhexyl2,5-di(perbenzoate), tert-butyl-di(perphthalate),tert-butylperoxy-2-ethyl hexanoate; and1,1-dimethyl-3-hydroxybutylperoxy-2-ethyl hexanoate; and peroxycarbonates, such as di(2-ethylhexyl) peroxy dicarbonate,di(n-propyl)peroxy dicarbonate, and di(4-tert-butylcyclohexyl)peroxydicarbonate. The peroxides can be used neat or in a diluent medium,having an active concentration of from 0.1 to 6.0 pph, preferably from0.2 to 3.0 pph.

According to this method, the propylene polymer material, preferably inparticulate form, at a temperature of from about 60° C. to 125° C. istreated with from 0.1 to 6.0 pph of a free-radical polymerizationinitiator described above. The polymer material is treated with about 5to 240 pph of a grafting monomer at a rate of addition that does notexceed 4.5 pph per minute at all addition levels of 5 to 240 pph of themonomer, over a period of time which coincides with or follows theperiod of treatment with the initiator. In other words, the monomer andinitiator may be added to the heated propylene polymer material at thesame time or the monomer may be added 1) after the addition of theinitiator has been completed, 2) after addition of the initiator hasstarted but has not yet been completed, or 3) after a delay time or holdtime subsequent to the completion of the initiator addition.

After the propylene polymer material has been grafted, the resultantgrafted propylene polymer material, while still maintained in asubstantially non-oxidizing environment, is treated, preferably byheating at a temperature of at least 120° C. for at least 20 minutes, todecompose any unreacted initiator and deactivate residual free radicalstherein. Any unreacted grafting monomer is removed from said material,either before or after the radical deactivation, or at the same time asdeactivation. The styrenic grafted propylene polymer material, component(b), is present in the amount of from 30 to 75%, based on the totalcomposition, except in the compositions containing optional component(d). When component (d) is present, component (b) is present in theamount of 5 to 30%, and preferably from 10 to 25%, based on the totalcomposition. In either case, the styrenic grafted propylene polymermaterial has from 5 to 70% of the styrenic monomer grafted or graftpolymerized thereto, and preferably from 10 to 55%.

The elastomeric block copolymers employed in the molding compositions ofthe present invention have at least two monoalkenyl aromatic polymerblocks, preferably polystyrene blocks S, and at least one hydrogenatedconjugated diene block, preferably hydrogenated isoprene (EP) orhydrogenated butadiene (EB), in a linear arrangement. The blockcopolymers are phase separated with weak monoalkenyl aromatic domainsresulting from limiting the structure of the block copolymer andlimiting the peak molecular weight of the monoalkenyl aromatic blocksfrom 4,500 to 10,000.

The monoalkenyl aromatic hydrocarbon-conjugated diene block copolymershave a total peak molecular weight of from 30,000 to 150,000 wherein theproportion of the monoalkenyl aromatic hydrocarbon monomer in themonoalkenyl aromatic hydrocarbon-conjugated diene block copolymer rangesfrom about 10% to about 25%.

The asymmetric S--EP--S' or S--EB--S' block copolymers have a peakmolecular weight from 30,000 to 150,000 and a styrene block(s) peakmolecular weight from 4,500 to 10,000, wherein the S block is greater inpeak MW than the S' block by at least 50%.

The tetrablock S--EP--S--EP' or S--EB--S--EB' block copolymers havetotal peak molecular weight from 45,000 to 90,000, wherein the Sendblocks has a peak molecular weight from 4,500 to 8,000, the EP or EBmidblock has a peak molecular weight from 35,000 to 55,000, the Smidblock has a peak molecular weight from 4,500 to 9,000, and the EP' orEB' endblock has a peak molecular weight that is 15% to 35%, preferably20% to 30%, of the peak molecular weight of the EP or EB midblocks.

The triblock S--EP--S block copolymer have total peak molecular weightsfrom 30,000 to 150,000 wherein the S blocks have peak molecular weightsfrom 4,500 to 7,000, preferably 5,000 to 5,500.

The most preferred block copolymer is an S--EP--S--EP' tetrablockcopolymer having peak molecular weights of 5,500 to 7,000 for the Sendblock, 40,000 to 50,000 for the midblock, and 6,000 to 8,500 for theS midblock, wherein the EP' endblock is from 20% to 30% of the peakmolecular weight of the EP midblock.

The preferred S--EP--S--EP' block polymers used in the invention havelow melt viscosity resulting from weak styrene domains that disassociatein the melt to give a one-phase melt instead of conventional two phasemelts.

The hydrogenated isoprene (EP) or hydrogenated butadiene (EB) blockshave a residual unsaturation less that 5%, preferably less than 2%. TheEP blocks have from 30% to 100% 1,4-addition and the EB blocks have from20% to 60% 1,4addition. The block copolymers are readily prepared byanionic polymerization and selective hydrogenation processes known inthe art.

The monoalkenyl aromatic hydrocarbon-conjugated diene block copolymer ispresent in the resin composition in an amount of from 1 to 15% by weightbased on the total polymer components, preferably from about 4 to 12% byweight.

Fillers and reinforcing agents, e.g. carbon black and glass fibers, aswell as inorganic powders such as calcium carbonate, talc, mica, andglass, may be included in the composition of the invention atconcentration levels up to about 80 parts by weight per 100 parts byweight of the polymer components. In addition to the economic benefitsuch fillers afford, greater stiffness and a higher heat distortiontemperature can be attained.

The components of the composition can be blended or admixed in anyconventional mixing apparatus, such as an extruder or a Banbury mixer.

HYPOTHETICAL EXAMPLES

Nine compositions of the invention are prepared by dry blending togetherin the appropriate ratios the graft copolymer, styrenic block copolymer,and the ethylene-propylene rubber. These mixtures are then added to thefeedthroat of a co-rotating twin screw extruder. The extrudertemperature is in the 200°-45° C. range, except for the melt zone, whichis 250°-265° C. The extruder is operated at a screw speed of 300 RPM.The blends are extruded into strands which are pelletized. Thepelletized blends are molded into test specimens via a reciprocatingscrew injection molding machine.

The graft copolymer of the above compositions is a graft copolymer of astyrene homopolymer grafted onto a propylene homopolymer backbone madeby the previously described peroxide-initiated graft polymerizationprocess wherein an oxygen-free mineral spirit solution oftert-butylperoxy 2-ethylhexanoate is sprayed onto the heatedpolypropylene (100° C.) and, after a short hold time, styrene is sprayedin. The following grafting conditions are used to prepare the graftcopolymers of (a) Examples 1 thru 3:1 pph peroxy ester, hold 15 minutes,54 pph styrene added at 1.64 pph/min., hold 3 hours at 100° C., then 4hours at 135° C. with nitrogen purge to deactivate and dry; (b) Examples4 thru 6: same as (a) except 2.35 pph peroxy ester, hold 10 minutes, and84.4 pph styrene added at 2.4 pph/min; and (c) Examples 7 thru 9: sameas (b) except 105 pph styrene added at 2.5 pph/min. All peroxy estervalues are on an active basis.

The propylene polymer used in the preparation of the graft copolymer isa finely divided porous propylene homopolymer (LBD-406A, commerciallyavailable from HIMONT Italia S.p.A.) in the form of generally sphericalparticles having the following characteristics: minimal melt flow rate(ASTM Method D 1238-82, Condition L) 8 dg/min; intrinsic viscosity(method of J. H. Elliott et . al., J/Applied Polymer Sci. 14, 2947-2963(1970)-polymer dissolved in decahydronaphthalene at 135° C.) 2.4 dl/g;surface area (B.E.T.) 0.34 m² /g; weight average diameter 2.0 mm; andpore volume fraction (mercury porosimetry method) 0.33. More than 90% ofthe pores in the porous particles are larger than 1 micron in diameter.

The ethylene-propylene rubber (EPM) used in Examples thru 9 has anethylene/propylene weight ratio of 57/43 and an intrinsic viscosity of3.37 dl/g.

Table 1 described the various styrenic block copolymers (S--EP--S,S--EP--S--EP', and S--EB--S') evaluated in the above described examples.

Table II shows the blend compositions described above.

                                      TABLE I                                     __________________________________________________________________________            Peak Molecular                                                                         Polystyrene                                                          Weight   Content (%)                                                                          Description                                           __________________________________________________________________________    S--EP--S                                                                              62,000   18     5,600--51,000--5,600                                  S--EP--S--EP                                                                          64,600   23     6,700--42,100--8,100--7,700                           S--EB--S'                                                                             53,000   15     5,000--45,000--3,000                                  __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________                Ex 1                                                                             Ex 2                                                                             Ex 3                                                                             Ex 4                                                                             Ex 5                                                                             Ex 6                                                                             Ex 7                                                                             Ex 8                                                                             Ex 9                                      __________________________________________________________________________    Polystyrene in Graft                                                                      31 31 31 42 42 42 54 54 54                                        Copolymer (% w)                                                               Graft Copolymer (% w)                                                                     85 85 85 85 85 85 85 85 85                                        S--EP--S (% w)                                                                            15       7.5      7.5                                             S--EP--S--EP (% w)                                                                           15       7.5      7.5                                          S--EB--S' (% w)   15       7.5      7.5                                       EPM (% w)            7.5                                                                              7.5                                                                              7.5                                                                              7.5                                                                              7.5                                                                              7.5                                       __________________________________________________________________________

What is claimed is:
 1. An improved rigid thermoplastic composition, wherein the composition comprises from 60% to 95 % by weight of the polymer components of a graft copolymer comprising about 10 % to 65 % by weight of the graft copolymer of a polystyrene grafted onto a backbone of polypropylene, and from 40 % to 5 % by weight of the polymer components of a rubber component comprising from 20 %-100 % of the rubber component of one or more elastomeric block copolymers, wherein the improvement comprises selection of a selectively hydrogenated block copolymer having the structure styrene-hydrogenated isoprene-styrene-hydrogenated isoprene (S--EP--S--EP'), sytrene-hydrogenated butadiene-styrene-hydrogenated butadiene' (S--EB--S--EB'), wherein the block copolymer has a total peak molecular weight from 45,000 to 90,000, the S endblock has a peak molecular weight from 4,500 to 8,000, the EP or EB midblock has a peak molecular weight from 35,000 to 55,000, the S midblock has a peak molecular weight from 4,500 to 9,000, and the EP' or EB' endblock has a peak molecular weight that is 15% to 35% of the peak molecular weight of the EP or EB midblock.
 2. The composition of claim 1, wherein the block copolymer has the block structure styrene-hydrogenated isoprene-styrene-hydrogenated isoprene' (S--EP--S--EP'), wherein the peak molecular weight of the EP' blocks is 20 % to 30 % of the peak molecular weight of the EP blocks.
 3. The composition of claim 2, wherein the peak molecular weight of the S endblock is from 5,500 to 7,000, the peak molecular weight of the EP midblock is from 40,000 to 50,000, and the peak molecular weight of the S midblock is from 6,000 to 8,500. 